Each year, approximately 25% of US teens sustain physical, psychological, or sexual abuse by dating partners. Many victims of teen dating violence (TDV) experience a host of devastating consequences, including acute and chronic mental and physical health problems, suicidality, delinquency, risky sexual behavior, substance abuse, and academic failure. Moreover, perpetrators of TDV are more likely to perpetrate other forms of violence and are at increased risk for continuing intimate partner violence in adulthood. Although TDV is a pervasive problem, it is largely preventable. One efficacious program, the Fourth R, integrates the promotion of healthy relationship skills and prevention of TDV into existing school curricula. Fourth R components are aligned with state and federal curriculum requirements in health, thus minimizing time and financial burdens placed upon resource-strapped schools. It includes both classroom activities and school-level components in which teachers receive specialized training on teaching about healthy relationships and students form safe school committees. Fourth R is used in over 5,000 schools in Canada, as well as several sites throughout the United States and Europe, and multiple systematic reviews and national agencies have identified it as one of only a few promising TDV prevention programs. Indeed, Fourth R has been shown to reduce physical TDV perpetration, increase relationship and peer resistance skills, and increase condom use. The effectiveness of Fourth R has been especially pronounced among high-risk youth. Developing an efficacious prevention program such as the Fourth R is only part of the equation. Implementing the program in real-world settings with fidelity and sustainability is the other, perhaps more difficult, part. Thus, prior to the Fourth R undergoing widespread dissemination and implementation, it is crucial to understand how factors at the school, teacher, and student levels affect program feasibility, fidelity, and sustainability. Also of interest is how this Canadian program is translated and adapted for use in US urban, suburban, and rural high schools. We propose to train teachers to implement Fourth R with 4 successive cohorts of 9th-grade students in 10 ethnically, economically, and geographically diverse high schools in Texas. The primary aims of this implementation study, which will use a mixed methods design, are to (1) assess initial feasibility, quality, acceptabilit, engagement, adaptation, and satisfaction with Fourth R, (2) examine school-, teacher-, and student-level barriers to high-fidelity implementation and correlates of low fidelity to the intervention protocol, (3) determine sustainability and impact of experience on implementation by assessing changes over the course of the study in fidelity, feasibility, acceptability, satisfaction, and engagement, and (4) evaluate effects of initial implementation fidelity and change over time on students' changes in attitudes, knowledge, skills, intentions, and behavior related to teen dating violence.